MLA
Henry, J. Letter from Henry J. to "Dear Cousin" - September 16, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 55. https://digital.library.emory.edu/purl/84302v6xm2-cor.
APA, 6th edition
Henry, J. (1861). Letter from Henry J. to "Dear Cousin" - September 16, 1861 (Box 6, Folder 55) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/84302v6xm2-cor.
Chicago
Henry, J. “Letter from Henry J. to ‘Dear Cousin’ - September 16, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 6, Folder 55. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/84302v6xm2-cor.